In ESXi 5.1 you could use the command ethtool -i vmnicX to get the firmware and driver version for the Network interface card. In 5.5 you get an error: Can not get control fd: No such file or directory.
To get the firmware and driver version use the following command in 5.5:
esxcli network nic get -n vmnicX
To get a list of nics:
esxcli network nic list